An architecture for high-performance direct conversion radios is disclosed in co-pending U.S. patent application Ser. No. 10/440,026 entitled “CHOPPER-DIRECT-CONVERSION (CDC) RADIO ARCHITECTURE” and filed May 16, 2003, the content of which is incorporated herein by reference. The architecture disclosed employs a phase-alternating mixer (PAM) that outputs a double-sideband (DSB) signal for use throughout the receiver circuitry. Such double-sideband signal paths eliminate the use of baseband frequencies that are generally problematic in large-scale circuit integration. In addition, problems associated with direct-conversion, such as local oscillator (LO) leakages, direct current (DC) offsets, low frequency noise and the like are all eliminated by relocating the local oscillator frequency away from the radio frequency (RF) employed for transmission.
When harmonic or alias signals are presented to the input of the radio, these unwanted signals interfere with the desired signal and degrade the quality of reception. To improve receiver quality, a surface acoustic wave (SAW) band select filter might be employed before the phase-alternating mixer to reject the unwanted signals. However, a better approach would be to design both the harmonic and alias rejection functions into the basic mixer circuitry.
There is, therefore, a need in the art for a harmonic rejection circuit that may be integrated into any type of mixer circuit, such as a phase-alternating mixer of the type described above.